Sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow

Abstract

An experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action. Wave‐induced oscillatory flow conditions near the seabed were simulated at full scale (1:1) in a new large oscillating water tunnel. Two sets of experiments (series A and B) were carried out. During series A, bed forms and wave‐cycle averaged suspended sediment concentrations were measured under sinusoidal waves. Series B focused on measurement of the wave‐cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows (upper shoreface conditions) and was aimed at the verification of quasi‐steady formulas for the description of cross‐shore sediment transport.Bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness. Only the measured ripple dimensions under asymmetric waves (series B) showed good agreement with Nielsen's (1979) relations. For most of the series B experiments the bed was plane (sheet flow), the net sediment transport was directed “onshore” and the measured transport rates showed a strong correlation with the velocity moment < U3 >. An empirical quasi‐steady transport model is proposed which is based on the new tunnel data and other existing data sets. The limitations of the quasi‐steady model approach became clear in the rippled‐bed regime and through the presence of a consistent influence of the wave period in plane‐bed conditions. In rippled‐bed conditions the suspended concentration profiles followed a negative exponential distribution. For most of the experiments the measured concentration decay length showed a linear relation with the ripple height. In plane‐bed/sheet flow conditions the measured suspended concentration profiles followed a negative power function. The power or concentration decay parameter was constant (α ≃ 2.1) for a wide range of velocity conditions. It is suggested that the mobile bed (sheet flow layer) has a strong damping effect on the mixing of suspended sediments (turbulence damping).